Phylum Mollusca – From Intuition to Precision
Think of the animals you meet in daily life that are soft, often have a shell, and move slowly. A snail in the garden, a clam buried in the sand, an octopus in the sea. They look very different, but they share a deep body plan. That is the intuition: Mollusca is the phylum of soft-bodied animals, usually protected by a hard shell, that glide on a muscular foot.
The name itself comes from Latin mollis = soft. These are the second largest phylum in the animal kingdom (after arthropods), with over 85,000 living species. They are mostly aquatic, especially marine, but some live on land.
The Three-Part Body Plan
Every mollusc, no matter how weird, is built on the same three-part chassis:
- Muscular foot – used for locomotion (creeping, burrowing, or jet propulsion in squids).
- Visceral hump – the mass of internal organs (digestive, circulatory, reproductive) sitting above the foot.
- Mantle – a fleshy fold of skin that covers the visceral hump and secretes the calcareous shell (if present).
The space between the mantle and the visceral hump is the mantle cavity, which houses the gills (or a lung in land snails) and the openings of the excretory and reproductive systems.
In octopuses and squids, the shell is reduced or absent. The mantle becomes muscular and forms the body wall.
The Radula – A Unique Feeding Tool
Most molluscs possess a radula: a ribbon-like, toothed structure in the mouth that works like a file or a rasp. It scrapes food particles (algae, plant matter) off surfaces. This is a defining feature of the phylum — no other animal group has a radula.
Bivalves (clams, oysters) have lost the radula. They are filter feeders, so they do not need to scrape. Do not assume every mollusc has one.
Key Features at a Glance
| Feature | Description |
|---|
| Body symmetry | Usually bilateral (some gastropods show torsion, making it asymmetrical) |
| Coelom | Reduced, mostly around the heart and gonads |
| Digestive system | Complete (mouth → anus), with a radula in most |
| Circulatory system | Open (except cephalopods, which have a closed system) |
| Respiration | Gills (ctenidia) in aquatic forms; a lung-like mantle cavity in terrestrial snails |
| Excretion | One or two nephridia (kidney-like organs) |
| Nervous system | Ganglia connected by nerve cords; sense organs (eyes, tentacles) well developed in cephalopods |
| Reproduction | Mostly sexual, with external or internal fertilisation; many have a free-swimming larval stage (trochophore or veliger) |
The Major Classes You Need to Know
| Class | Common name | Key features | Example |
|---|
| Gastropoda | Snails, slugs | Asymmetrical body due to torsion; one-piece coiled shell (or absent); head with tentacles and eyes | Pila (apple snail), Helix (garden snail) |
| Bivalvia | Clams, oysters, mussels | Two hinged shells; no radula; filter feeders; foot is hatchet-shaped for burrowing | Unio (freshwater mussel), Mytilus (marine mussel) |
| Cephalopoda | Squid, octopus, cuttlefish | Shell reduced or absent; foot modified into arms/tentacles; head well developed with large eyes; closed circulatory system; jet propulsion | Octopus, Sepia (cuttlefish), Loligo (squid) |
| Polyplacophora | Chitons | Eight overlapping shell plates; foot broad and flat; radula present; marine, cling to rocks | Chiton |